Reactivity and Statistical Factors

Lets consider the chlorination of butane again in greater detail. We see that there are four ways to get 2-chlorobutane and six ways to obtain 1-chlorobutane. The ratio of 2-chlorobutane to 1-chlorobutane therefore equals the relative rate of abstraction of a secondary hydrogen atom times 4 divided by the relative rate of abstraction of a primary hydrogen atom times 6. 

2-chlorobutane forms in preference to 1-chlorobutane because a single secondary hydrogen atom is abstracted 3.9 times as rapidly as a single primary hydrogen atom. 

A similar analysis for the chlorination of 2-methylpropane illustrates the reactivity per hydrogen atom even more dramatically. Recall that the primary product predominates in a 63 37 percent ratio. 

In this case, the major product does not result from abstraction of the more reactive hydrogen atom. A tertiary hydrogen atom is abstracted at a faster rate than a single primary hydrogen atom, but more primary product forms because there are enough primary hydrogen atoms to offset the difference in relative reactivities. 

The order of reactivity per hydrogen atom in the chlorination of alkanes illustrates the low 

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